Ships transporting almost a fifth of Iran’s oil exports in the second half of last year either turned off their radio-signal tracking systems or gave misleading information about the origin of their cargo, red flags for governments seeking evidence of evasion of international sanctions against Tehran.

Some 47 of 55 ships carrying Iranian oil products from Iran to the United Arab Emirates for two U.A.E.-registered companies failed to emit signals from the system that transmits their position and course, for part or all of...